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1 .Listen to part of a lecture in a biology class.
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2 .So we\'ve been talking about cetaceans, marine mammals like whales and dolphins.
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3 .Are there any questions before we leave the topic?
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4 .Yeah, I\'ve been reading about these times when cetaceans are found stuck on ocean beaches. They call it Stranding. Right, right? Stranding.
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5 .I mean, like sometimes 100 or more animals are found stranded on beaches.
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6 .So what causes that?
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7 .Yes, strandings are often in the news, especially mass stranding like you mentioned.
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8 .We\'re not really sure why this happens, but several hypotheses have been proposed.
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9 .The first hypothesis has to do with changes in the direction of Earth\'s magnetic field in parts of the ocean floor,
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10 .Earth\'s magnetic field can change direction.
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11 .I thought it\'s just oriented south to north.
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12 .It generally is. But in some areas, the magnetic fields affected by local conditions or by geological processes on the ocean floor.
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13 .But getting back to cetaceans
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14 .in the outer layer of the cetacean brain,
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15 .there\'s a substance called magnetite.
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16 .We think that magnetite makes it possible for cetaceans to sense Earth\'s magnetic field
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17 .and so figure out which way to swim.
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18 .Researchers wondered if there could be a link between strandings and areas near coastlines where the magnetic field suddenly changes direction.
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19 .And one European study did indeed show a connection between regions with magnetic variations and mass strandings.
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20 .Another hypothesis that\'s been put forth
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21 .has to do with the shape of the coastline.
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22 .Time and again,
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23 .mass strandings occur in areas where the coastline juts out into the sea
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24 .with deep water on either side. You mean, like
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25 .Cape Cod in the northeastern United States.
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26 .You know the way it\'s a peninsula, like a long arm extending out from the mainland.
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27 .I\'ve seen stuff on the news about strand of whales there.
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28 .Cape Cod is a good example.
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29 .Areas like this are risky for cetaceans, because they\'re more accustomed to deep water.
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30 .So when the animals suddenly find themselves in shallow water,
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31 .or if they get lost in a storm,
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32 .they can end up on the beach.
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33 .Weather can cause strandings, even when it doesn\'t seem too bad.
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34 .That\'s what sometimes happens with upwellings.
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35 .Upwellings form as a result of wind blowing steadily in the same direction,
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36 .the wind displaces the warmer surface water in an area,
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37 .allowing colder, deeper water to rise to the surface,
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38 .which is what we call upwelling.
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39 .This deeper water is particularly rich in nutrients, because after sea animals die, they usually sink to the ocean floor where they decompose,
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40 .so a layer of nutrients builds up on the sea bottom.
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41 .But upwellings bring those nutrient rich areas to the surface,
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42 .and these areas attract many feeders.
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43 .Wherever an upwelling occurs, animals will follow.
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44 .But when certain types of winds are present,
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45 .upwellings can be pushed closer to the coast than where they normally occur.
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46 .Animals who follow these zones could find themselves in unfamiliar, shallower water,
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47 .where they are at a high risk of getting stranded.
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48 .I thought the whales that get stranded are usually sick.
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49 .Are you saying strandings are always the result of environmental or geographical causes?
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50 .No, you\'re right.
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51 .Cetacean strandings are sometimes caused by sickness too.
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52 .But even in those cases, there\'s often more to the story.
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53 .In mass strandings, only one animal may actually be sick,
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54 .often an older member of the group that others may have seen as a leader.
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55 .You see many cetacean species have a very stable group structure
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56 .and rarely abandon other group members.
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57 .So if a leader were to become sick or disoriented and move dangerously close to land,
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58 .other group members would be likely to follow
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59 .some whale species form familial groups that stay together for life.
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60 .It\'s these species tend to strand on masse.
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